Alkali metal chlorites are well known precursors of chlorine dioxide with a wide range of applications, mainly in water treatment, pulp bleaching and textile bleaching. Chlorites are prepared typically by the reaction of chlorine dioxide, a reducing agent and an alkali. An exhaustive discussion of various preparative methods for chlorite synthesis can be found in the basic textbook entitled: "Chlorine Dioxide. Chemistry and Environmental Impact of Oxychlorine Compounds" by W. J. Masschelein, 1979, pp. 130 to 145.
Various improvements to the basic concept of reacting chlorine dioxide with the reducing agent and alkali to form chlorite are disclosed in the U.S. Patents discussed below.
U.S. Pat. Nos. 2,092,944 and 2,092,945 (Vincent) disclose the preparation of water soluble chlorites by reacting chlorine dioxide with an alkaline solution containing sulfur or a carbonaceous reducing agent.
U.S. Pat. No. 2,194,194 (Cunningham) discloses the use of metallic reducing agents for the preparation of chlorites.
U.S. Pat. No. 2,332,180 (Soule) discloses the use of hydrogen peroxide and alkali metal bicarbonate in chlorite synthesis. The same reducing agent is disclosed in the U.S. Pat. No. 2,616,783 (Wagner), related to the preparation of solid chlorite.
U.S. Pat. No. 3,101,248 (Hirschberg et al) discloses a process for chlorite synthesis involving the use of various alkali metal and alkaline earth metal amalgams as reducing agents.
U.S. Pat. No. 3,450,493 (Du Bellay et al) discloses a method for the manufacture of alkali metal chlorites, employing a continuous monitoring of redox potential and pH for correct process control.
U.S. Pat. No. 3,828,097 (Callerame) discloses a process for the preparation of chlorous acid, involving the use of nitrite in a column containing a cation exchange resin.
U.S. Pat. No. 4,087,515 (Miller) discloses the use of alkali metal amalgams as reducing agents whereby the process is carried out under an atmosphere of nitrogen gas to prevent an excessive build-up of chlorine dioxide.
U.S. Pat. No. 5,597,544 (Barber et al) and U.S. Pat. No. 5,639,559 (Mason et al) disclose a gas phase reaction between chlorine dioxide and reducing agent whereby the resulting chlorous acid is subsequently reacted with aqueous solution of the base, such as hydroxide, carbonate or bicarbonate to form chlorite in high yield.
A major drawback of all of the above described processes is a high content of certain impurities, particularly carbonates and bicarbonates, in the final product. According to the published literature (see, for example, previously cited Masschelein, p. 131, lines 10 and 11) a typical, commercial 80% sodium chlorite product generally contains about 5% sodium carbonate.
Such a high level of carbonates is detrimental at the point of use of alkali metal chlorite, in particular when chlorite is converted to chlorine dioxide to be used for water disinfection or pulp bleaching. The presence of carbonates causes the formation of scale in the equipment employed for chlorine dioxide generation, resulting in higher operating costs and troublesome maintenance. While there are known methods for the purification of sodium chlorite from the carbonate impurity, they are very costly and often they create more problems than they solve. For example, a carbonate removal method based on the precipitation of lead carbonate (see Masschelein, p. 138) may result in the contamination of chlorite with highly poisonous lead compounds, rendering the product unsuitable for water treatment applications.
There is a need, therefore, to develop an economical process enabling the manufacture of alkali metal chlorite with a very low carbonate content, thus eliminating the costly purification step of the final product.